It is well known that vinyl aromatic compounds such as monomeric styrene, lower alkylated styrene, e.g., alphamethyl styrene, and divinylbenzene polymerize readily, and furthermore, that the rate of polymerization increases with increasing temperature. Inasmuch as styrene and divinylbenzene produced by common industrial methods contain impurities, these compounds must be subjected to separation and purification processes in order to be suitable for most types of further industrial use. Such separation and purification is generally accomplished by distillation.
In order to prevent polymerization during distillation of vinyl aromatic compounds, various types of known polymerization inhibitors have been employed in connection with prior art distillation processes. For example, common inhibitors useful for inhibiting the polymerization of vinyl aromatics under distillation conditions include nitrated phenols. It is preferred, however, to purify vinyl aromatics by using vacuum distillation techniques, whereby these commonly employed inhibitors are rendered unsuitable in view of the fact that they are effective only in the presence of oxygen. The partial pressure of oxygen in a vacuum distillation column is accordingly too low for these conventional inhibitors to be effective. Sulfur is perhaps the polymerization inhibitor most commonly employed to inhibit polymerization of vinyl aromatic compounds during distillation, since sulfur does provide effective inhibition in the absence of oxygen. While sulfur provides a reasonably effective inhibitor, its use in distillation processes results in one very significant disadvantage, namely, there is formed in the reboiler bottoms of the distillation column, a valueless waste material which is highly contaminated with sulfur. This waste material furthermore represents a significant pollution or waste removal problem.
In a typical distillation process for vinyl aromatic compounds utilizing a polymerization inhibitor, the mixture of vinyl aromatic to be distilled is generally contacted with the chemical polymerization inhibitor prior to being subjected to distillation conditions in the distillation apparatus. It remains as a significant problem today that the amount of polymer formed in the distillation apparatus and in the high purity product recovered therefrom is substantially higher than desired, and occasionally, that complete polymerization occurs inside of the distillation apparatus.
Although many compounds are effective for inhibiting the polymerization of vinyl aromatic compounds under differing conditions, e.g., storage, other purification techniques, etc., for a number of reasons which are not entirely understood in view of the diverse and unpredictable results obtained, only extremely few of these compounds have proved to be of any utility for inhibiting vinyl aromatic polymerization under distillation conditions, particularly under vacuum distillation conditions. In addition, certain compounds which are useful for inhibiting polymerization of one type of vinyl aromatic compound, for example, styrene, have proved to be essentially ineffective for inhibiting polymerization of another species of vinyl aromatic compound, for example, divinylbenzene. A limited number of nitroso compounds have proven to be effective for inhibiting polymerization of styrene monomer during distillation. For example, N-nitroso phenylhydroxylamine and p-nitroso-N,N-dimethylaniline are reasonably effective inhibitors for the distillation of styrene, although they are not particularly soluble in styrene monomer. On the other hand, N-nitroso diphenylamine disclosed in U.S. Pat. No. 3,816,265, assigned to the assignee of the present application has been demonstrated to be a particularly effective polymerization inhibitor under vacuum distillation conditions for both styrene and divinylbenzene, whereas N,N-nitrosomethylamine has been found to be an excellent polymerization inhibitor for styrene under vacuum distillation conditions. One of the most effective inhibitor systems known for divinylbenzene comprises a mixture of sulfur and N-nitroso phenylhydroxylamine
U.S. Pat. No. 3,786,110 teaches that in a process for the pyrolysis of hydrocarbons or substituted hydrocarbons in a high temperature zone (about 1200.degree. C.) followed by rapid cooling in a quench zone, the serious problem of formation of hard deposits on the interior of apparatus downstream from the quench zone is solved by adding asphaltenes to the pyrolysis product in or just after the quench zone. The asphaltenes provide sites for polymer molecules to attach themselves; also asphaltenes are capable of reacting with the highly reactive polyolefins to form low molecular weight polymers, thus preventing the formation of high molecular weight products. In contrast, in the process of the present invention and the applicable commercial applications, the formation of even low molecular weight polymers is not acceptable.
U.S. Pat. No. 4,376,678 to Partos teaches the utilization of 2,2-bis(3,5-dinitro-4-hydroxyphenyl) propane as a polymerization inhibitor. The Partos patent contains a listing and discussion of several U.S. and foreign patents and document which relate to polymerization inhibitors. Each of the patents or articles relates to a specific chemical compound utilized as a polymerization inhibitor.
As can be seen from the above, a readily available and less costly polymerization inhibitor would be desirable.